The present invention relates to electrically heated catalytic converter modules for automotive exhaust gas treatment applications. More particularly, the invention relates to an improved design for an assembly for such modules specially adapted for use with metallic honeycomb heaters for electrically heated catalytic converter modules.
The use of axially assembled enclosures containing ceramic honeycomb-supported catalysts for the treatment of automotive exhaust gases is known. U.S. Pat. No. 4,207,661 to Mase et al., for example, describes such an assembly comprising forward and rear case halves into which a ceramic catalytic converter substrate can be inserted. Included in the assembly are front and rear supporting members composed of a resilient material for supporting the catalytic converter substrate within the enclosure while shielding it from mechanical shocks.
Axial assembly in the manner of the above patent is advantageous in that the number of components required to securely encase the catalytic substrate within the shielding metal container is relatively small, and in that axial compression of the resilient supports for the ceramic honeycomb within the enclosure to improve the mechanical shock resistance of the assembly is more easily effected. However, one disadvantage of many of these enclosures is that the compression levels attainable are not variable, but are instead constrained by the relative dimensions of the can and honeycomb. And, as in U.S. Pat. No. 5,250,269, some designs do not utilize axial compression at all.
More complex axial constructions, such as that described in U.S. Pat. No. 4,347,219 to Noritake et al., can overcome some of these difficulties. However, the assemblies of the latter patent require a relatively large number of parts, and thus a large number of associated assembly and welding steps. Further, many of these assemblies are designed to contain relatively long honeycomb bodies and, more importantly, make no provision for gas-tight electrode feed-throughs since they are designed for catalyst substrates rather than flow-through heaters.
It has so far not been feasible to directly adapt canning assemblies for ceramic catalytic converters to the mounting of metal honeycomb heaters. Among the problems encountered in this regard is the fact that metal honeycombs for the electrical heating of exhaust gases or other fluids require electrical contacts for electrically energizing the honeycombs. These contacts typically comprise relatively heavy electrodes which pass through the walls of the metal containers used to enclose and protect the honeycombs.
Electrode pass-through structures have presented a design challenge in that they must be both gas-tight and electrically insulating, to prevent exhaust leakage as well as grounding of the electrodes to the containers.
Enclosures for electrically heated metal honeycombs for automobile exhaust use must also provide physical protection adequate to enable the heaters to meet government mandated standards for maximum allowed levels of non-methane hydrocarbons, CO, and nitrogen oxides for up to 100,000 miles of automobile use. This can involve up to 50,000 engine starting cycles and requires sustained heater integrity under severe thermal cycling, extreme temperatures, and high temperature vibration.
The problem of physical protection is aggravated by the fact that metal honeycomb designs useful for electrical heaters are somewhat lower in crushing strength and durability than their ceramic counterparts, especially at elevated engine exhaust temperatures. This places a premium on the effectiveness of the enclosure design for insulating the metal honeycomb structure from mechanical shock damage.
Finally, a number of exhaust system designs incorporating honeycomb exhaust heaters require the close mounting of a metallic or ceramic honeycomb-supported auto exhaust catalyst, called a light-off catalyst, against the heater. This is required so that electrically generated heat energy can be efficiently transferred from the metal honeycomb heater to the light-off catalyst, in order that rapid heating of the catalyst to operating or so-called light-off temperatures can be achieved.
Previous efforts to meet the various requirements for supporting heating elements in automotive exhaust systems have involved a clam-shell assembly packaging approach, wherein top and bottom half-shell enclosure sections having electrode pass-through holes or recesses have been radially compressed together over the honeycomb heater and associated insulation material. These enclosure portions are then welded together. Such enclosures have proven to be complex to assemble, and the parts are costly to fabricate. In addition, even small variations in the shape or size of the parts can result in large variations in the preloading forces under which the honeycomb heater is protected from vibration damage in the enclosure.